1. Field of the Invention
The present invention relates to a host digital terminal HDT, and more particularly, to a host digital terminal capable of transmitting large amounts of data, switching many units among the units to make the units mutually interact, wherein each one of the units performs a predetermined function respectively.
2. Description of the Related Art
Recently, according to a variety of the class and explosive increases of the demands of data communication services, subscribers accustomed to the data transmission of low and middle speeds, such as voice data transmission, have become to request a wide area network WAN capable of providing a high-speed digital data transmission service, such as multimedia data transmissions.
Therefore, many technologies are developed to provide such a high speed digital data service. Two standards capable of providing a high-speed digital data transmission service among the developed technologies are a fiber to the curb FTTC technology and a fiber to the home FTTH technology.
The FTTC technology is applied when optical fiber cables are installed from the public network to the front door of each home of the subscribers. In this case, the curbs are located in each of the densely inhabited districts of the subscribers respectively, and optical transmissions are performed through the optical fiber cables from a switching apparatus in the public network to the curbs. Moreover, data is transmitted through conventional telephone lines from the curb to each of the corresponding subscribers.
On the other hand, the FTTH technology sets the curb in each home of the subscribers, respectively.
A Fiber Loop Carrier FLC system connects a subscriber access network to a token-ring or a star topology, so that the subscriber access network embody such as FTTC or FTTH technology efficiently.
FIG. 1 is a schematic network configuration illustrating such a conventional FLC system.
The conventional FLC system, as shown in FIG. 1, includes a host digital terminal HDT 10 and many optical network units ONU 20. The HDT 10 is installed within a Central Office CO (not shown) that is a control manager of the subscriber access network. Further, many of the ONUs 20 connected to the HDT 10 in a star topology are installed within each of the subscribers, such as the curbs in each home and company.
In this case, the HDT 10, connected to a public switched telephone network PSTN 2 and an asynchronous transfer mode ATM 1 through optical fiber cables, transmits a normal telephone data, an internet data, and an interactive video data from the PSTN 2 or the ATM 1 to each of the ONUs 20 through optical fiber cables. Moreover, the HDT 10 inversely transmits those data from each of the ONUs 20 to the PSTN 2 or the ATM 1.
Each of the ONUs 20 makes the optical data signal transmitted from the HDT 10 to be inverse-multiplexed to provide a high-speed asynchronous transfer channel, having a transmission speed upward of 16˜24 Kbps (kilobits per second) and downward transmission speed of 1.5˜6 Mbps or 2˜8 Mbps (megabits per second) in the case of E1 (European digital line interface), for each of the subscribers 30. In this case, each of the ONUs 20 utilizes twisted pair transmission lines for transmitting those data to each of the subscribers 30.
As described in the above statements, the HDT 10 has a role of backbone networks to the subscriber access network for connecting the PSTN 2 and the ATM 1 to the ONU 20.
As shown in FIG. 2, the conventional HDT 10 includes a network interface unit NIU 12, an optical transfer unit OTU 13, a main control unit MCU 11, and a common bus 14 within a shelf.
The NIU 12 interfaces with the ATM and PSTN 1 and 2, and many of the OTUs 13 interface with the corresponding ONUs 20. Additionally, the MCU 11 controls all of the data flows, such as mutual interactions among those units or operations and maintenance for ATM cells, while all of those units are mutually interacted through a common bus 14.
Further, two NIUs 12 are installed in a shelf Each of the NIUs 12, including 4 ports of a synchronous transfer module level 1 STM-1, receives 4 STM-1 signals having a transfer rate of 155.52 Mbps in each signal from the ATM 1, and translates the overhead included in the STM-1, or vice versa. Sequentially, after extracting data of the ATM cells, the NIU 12 sends the extracted ATM data to the common bus 14, or vice versa.
On the other hand, 8 OTUs 13 are installed in a shelf, and an OTU 13 is connected to two ONUs 20. Therefore, the HDT 10 equips 16 ONUs 20.
The OTU 13 receives the ATM cell data transmitted from the common bus 14, and converts the received ATM data to an optical signal of synchronous transfer module level 4 STM-4 having a transfer rate of 622.08 Mbps to transmit the ATM data to the corresponding ONU 20, or vice versa.
In summary, the conventional HDT 10, includes the NIU 12 having 4 STM-1 ports, for sending STM-1 signals to the ATM and PSTN 1 and 2 and for receiving the STM-1 signals from the ATM and PSTN 1 and 2, and 8 OTUs 13, wherein each of the OTUs 13 can connect to 2 ONUs 20. Additionally, the NTU 12 and the OTUs 13 interact with each other through the common bus 14.
However, the conventional HDT 10 has some difficulties in receiving additional requirements of the subscribers for a high speed data service, such as WAN or a multimedia based service, and exponential increases of the subscribers
First of all, sending and receiving the ATM cell data among the units 11˜13 in the conventional HDT 10, as shown in FIG. 2, are performed through the common bus 14. Therefore, the above described problems set limits in treating data traffics, and make it difficult to allow large transmissions of the data.
For example, when lots of ports in each of the units 11˜13 simultaneously request data transmissions of the ATM cells to the common bus 14, redundant data of the ATM cells are all mixed up to make the data collided mutually. To overcome the above mentioned problems, the common bus 14 having a large capacity can be applied. However, it is not efficient in view of an economic aspect, etc.
Therefore, an interactive service based on multimedia data of large capacity, such as a video conference, a video medical examination and treatment, and a video on demand VoD, is hardly provided to the many subscribers.
Secondly, it is difficult to establish an economic system expandable to accept additional subscribers due to the lack of capacity.
Furthermore, when a star topology is applied for embodying the FTTH easily by expanding many of the star nodes, such as an ATM passive optical network PON, or many ONUs 20 are applied for connecting many subscribers expansively, it is required that there be a backbone network for supporting the topology and lots of ONU 20 perfectly. However, the conventional HDT 10 hardly satisfies such requirements.